Large area solid state x-ray detectors are currently being developed in the x-ray art. Such a detector typically comprises a scintillating layer in contact with an array of photodiodes, organized in rows and columns, each with an associated FET switch. The scintillator converts x-ray photons to light photons. The array of photodiodes converts light photons to electrical signals. The photodiodes are initially charged by connecting them to a known stable voltage through the FET switches. Subsequently, the photodiodes are isolated by turning the FETs off. Upon exposure to x-rays, the scintillator produces light which discharges each photodiode in proportion to the x-ray exposure at the position of the diode. The diodes are then recharged by again connecting them to the known stable voltage. The charge used to restore the diode to its initial voltage is measured by a sensing circuit, and the value is digitized and stored. The resulting array of digital values comprises an x-ray image of the distribution of x-rays impinging on the detector.
Proposed x-ray detectors made with this technology will contain a large number, as many as several million, photodetector elements. In the course of manufacturing such an array, inevitably, a fraction of the elements will be defective. Fortunately, the use of perfect detector is not required for medical x-ray imaging. The minimum size of objects that can be clearly seen in a medical image is determined by the modulation transfer function (MTF) of the imaging system. For a large area solid state detector the factors contributing to MTF degradation are lateral spread of light photons and of secondary x-ray photons in the scintillator and the finite size of the pixel. The thickness and structure of the scintillator and the pixel size are designed so that the MTF of the imaging system is adequate to view the smallest objects of interest in an image. In particular, the pixel size is chosen so that objects of interest in the image spread their signal over more than one pixel. Therefore, provided the bad pixels are not aggregated in sizable clusters, the loss of information due to bad pixels is negligible. However, because the signal from a defective pixel is either independent of x-ray exposure or depends on x-ray exposure in a way that is much different from that of surrounding pixels, a defective pixel will have a value that stands out from its neighbors. If bad pixel values were left unaltered in the displayed image, they would interfere with the visualization of the rest of the image.
It is necessary, then, to have a means for identifying bad pixels and a means for changing bad pixel values to ones that will blend in with neighboring good pixel values.